This invention relates to a method for forming spherical particles of thermoplastic materials having relatively low-melting points, such as natural resins, synthetic resins, and the like, and also to apparatus for carrying out the same method.
Conventionally, a number of methods for forming spherical particles of thermoplastic materials have been available. They include dry processes in which thermoplastic particles are either suspended in a hot atmosphere for a predetermined period of time so as to form a fluidized bed or dropped into a heated tube, and wet processes in which a solute dispersed or dissolved in water or an organic solvent is sprayed in a hot atmosphere to evaporate the solvent and thereby obtaining spherical solute particles.
In the aforesaid dry processes, however, it is difficult to keep the particles in the specified heating space for a desired period of time under an individually separate condition. Especially when the particles are not greater than 100 .mu.m in diameter, they tend to fuse together into an agglomerate and/or stick to the vessel walls during the operation for forming them into spherical particles, resulting in a nonuniform degree of sphericity and an unduly decreased yield.
On the other hand, the wet processes (for example, the spray drying process) have the advantage of producing uniformly spherical particles over a wide range of particle diameter extending from several micrometers to several hundred micrometers. However, the solvent present in the sprayed particles must be evaporated almost completely before they can be collected. This poses the problems of requiring an immense drying space and hence an oversized apparatus; causing an increase of incidental facilities (due to the need for recovering the solvent) if the evaporated solvent is other than water; and involving risks of fire, toxicity, and the like.